U.S. patent number 4,758,304 [Application Number 07/028,246] was granted by the patent office on 1988-07-19 for method and apparatus for ion etching and deposition.
Invention is credited to John R. McNeil, Scott R. Wilson.
United States Patent |
4,758,304 |
McNeil , et al. |
July 19, 1988 |
Method and apparatus for ion etching and deposition
Abstract
The disclosure relates to a method and apparatus using ion
etching and ion assisted deposition to reform a surface of an
object, such as a large lens, from its existing topography to a
predetermined topography. The method comprises comparing the
existing topography of the surface of the object to the
predetermined topography. In one embodiment, the comparison can be
used to distinguish objects having surfaces which are readily or
economically reformable to the predetermined topography from those
which are not suitable for such reforming. The method novelly
utilizes an algorithm comprising image restoration. The ion etching
structure of the apparatus comprises an ion source grid which can
be used to provide an ion beam of a preselected spatial
distribution. The grid is constructed of a nonconducting, vacuum
compatible material, such as a ceramic sheet coated with a
conductive layer on each side. Apertures are drilled through the
grid in a selected pattern. The ion beam produced from a plasma
source when a suitable voltage is applied across the coatings has a
spatial distribution in accordance with the aperture pattern. In
one embodiment the coatings comprise discrete corresponding areas
on each surface and different voltages are appliable to each area
to further control beam spatial distribution. Ion assisted
deposition may be simultaneously performed under the algorithm to
add material to the surface in accordance with the desired
predetermined topography.
Inventors: |
McNeil; John R. (Albuquerque,
NM), Wilson; Scott R. (Albuquerque, NM) |
Family
ID: |
21842376 |
Appl.
No.: |
07/028,246 |
Filed: |
March 20, 1987 |
Current U.S.
Class: |
216/60; 118/50.1;
204/192.13; 156/345.39; 216/66; 216/67; 118/620; 204/192.11;
204/192.34 |
Current CPC
Class: |
H01J
37/3023 (20130101); C03C 23/0055 (20130101); H01J
37/3053 (20130101) |
Current International
Class: |
C03C
23/00 (20060101); H01J 37/302 (20060101); H01J
37/305 (20060101); H01J 37/30 (20060101); B44C
001/22 (); C03C 015/00 (); C23C 014/00 (); B05D
003/06 () |
Field of
Search: |
;156/626,643,646,654,345
;204/192.1,192.11,192.13,192.32,192.33,192.34,298 ;427/38,39
;118/728,50.1,620 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Powell; William A.
Attorney, Agent or Firm: Sopp; Albert Weig; Robert W.
Claims
We claim:
1. A method for ion etching a selected surface of an object from
its existing topography to a predetermined topography, the method
comprising the steps of:
(a) comparing the existing topography of the surface to the
predetermined topography,
(b) using an algorithm comprising image restoration, etching the
surface with an ion beam to remove selected material therefrom,
thereby causing the surface to reach the predetermined
topography.
2. The invention of claim 1 further comprising depositing material
on selected portions of the surface in accordance with the
algorithm to cause the surface to reach the predetermined
topography.
3. The invention of claim 1 wherein the predetermined topography is
symmetric.
4. The invention of claim 1 wherein the predetermined topography is
nonsymmetric.
5. The invention of claim 1 wherein the ion beam is a high current,
low energy, broad beam.
6. The invention of claim 1 wherein the ion beam has a current
between about 0.2 kV and about 2 kV, an energy between about 800 eV
and about 1500 eV and focuses to between about 2 and 5 centimeters
and larger in diameter.
7. The invention of claim 1 further comprising using surface
extrapolation to avoid edge effects.
8. The invention of claim 1 wherein said algorithm is: ##EQU4##
where at t(x,y) the ion beam is at each point (x,y) F(u,v) is the
orthogonal transform of a predetermined surface,
D(u,v) is the orthogonal transform of the desired surface
profile,
H(u,v) is the orthogonal transform of the surface material
modification profile,
F.sup.-1 is the inverse orthogonal transform of the forward
transform used to form F, D and H, and
.gamma. is the multivariate parameter.
9. The invention of claim 1 wherein the beam has a profile of
substantially Gaussian shape.
10. The invention of claim 1 wherein the beam emerges from a grid
comprising a plurality of discrete sources spaced in a
predetermined pattern.
11. The invention of claim 10 further comprising controlling the
spatial distribution of the ion beam.
12. The invention of claim 10 wherein said pattern comprises a
plurality of concentric circles.
13. A method of sorting unprocessed objects having a surface with a
topography suitable for surface etching to a predetermined
topography from those having surface topographies not suitable for
etching, the method comprising the steps of:
(a) comparing the existing topography of the object's surface
considered for etching with the predetermined surface topography
using an algorithm utilizing image restoration to determine whether
a selected object comprises a surface having an existing topography
which can be etched to the predetermined topography in accordance
with preselected criteria,
(b) accepting for etching those objects comprising surfaces which
can be etched within the preselected criteria, and
(c) rejecting those objects which do not comprise surfaces meeting
the criteria.
14. The invention of claim 13 wherein the criteria comprise time as
a criterion.
15. The invention of claim 13 further comprising the step of ion
etching the surfaces of the objects meeting the preselected
criteria in accordance with the algorithm.
16. The invention of claim 13 wherein the algorithm comprises
deblurring and image restoration calculations for the computation
of a beam dwell array.
17. An ion source grid apparatus for extracting ions from a source
plasma, said apparatus comprising:
a two surface sheet of nonconductive material;
a conductive layer disposed on each surface of said sheet;
a plurality of apertures penetrating both said conductive layers
and said sheet in a predetermined pattern for providing an ion beam
having a preselected spatial distribution; and
means for supplying electrical voltage across said layers.
18. The invention of claim 17 wherein said plurality of apertures
are disposed within a substantially circular central region and at
least one circle concentric thereto.
19. An ion source grid apparatus for extracting ions from a source
plasma, said apparatus comprising:
a two surface sheet of nonconductive material;
like conductive layers disposed on each surface of said sheet, each
layer comprising a plurality of separate and discrete areas, said
like layers comprising corresponding areas;
a plurality of apertures penetrating, within said discrete areas,
both said conductive layers and said nonconductive sheet in a
predetermined pattern for providing ion beam having a preselected
spatial distribution; and
means for supplying a selected electrical voltage across each said
corresponding area of said like conductive layers to provide an ion
beam of a desired spatial distribution.
20. The invention of claim 19 comprising means for controlling each
selected voltage to change said desired ion beam spatial
distribution in accordance with said voltages.
21. The invention of claim 19 wherein said corresponding layer
areas essentially comprise a substantially circular central area
and at least one circle concentric thereto.
Description
FIELD OF THE INVENTION
The invention relates to ion etching and ion assisted and other
deposition and more particularly to ion etching existing surface
topographies of objects to predetermined topographies in accordance
with an algorithm using image restoration and to an ion source
extraction grid apparatus providing selected ion beam spatial
distribution.
BACKGROUND OF THE INVENTION
Precision optical components having physical surface height
aberrations smaller than one wavelength of the incident radiation
are of strategic importance to the operation of many optical
systems. Such components are very expensive because of the enormous
investment of time and sophisticated equipment required to
fabricate and figure optical surfaces. Conventional methods of
fabricating and figuring optical surfaces involve grinding surfaces
into optics using abrasives. Although modern optic grinders have
better abrasives, tools and even equipment under computer control,
the physical process is essentially the same as it has been for
hundreds of years.
Ion etching, also known as ion sputtering and ion milling, has been
tried as an alternative process to abrasion. Ion sputtering is a
physical process in which an ion is caused to impinge upon a
surface of an object with sufficient energy to cause atoms or
molecules of the object to be liberated from its surface.
Sputtering has become popular in the semiconductor industry.
However, sputtering has not found use in modifying optical surfaces
because efforts to use sputtering for optic surfacing were severely
limited by the ion current from the ion sources available at the
time. One type of ion source used in such attempts is known as a
Cockraft-Walton accelerator. U.S. Pat. Nos. 3,548,189 to Meinel et
al and 3,699,334 to Cohen et al illustrate such ion sources in
their disclosed devices. Cockraft-Walton as well as other ion
accelerators used in such attempts are only capable of driving a
maximum beam current of a few hundred microamperes and produce
quite high ion energies, often a fraction of an MeV. Limitations
result from the fundamental design of such ion sources. For
example, such sources contain only a single aperture for ion
extraction. The ion current extractable from a single aperture s
proportional to the voltage applied to the aperture which in turn
determines the ion energy. The use of a single aperture as in the
prior art thus mandates that high voltage be applied to the ion
extraction aperture which results in high energy ions in order to
obtain an ion current on the order of a hundred microamperes. Due
to such limitations ion beam etching has been essentially
unworkable.
In the late 1970's the Kaufman ion source as disclosed in the
publication, Technology of Ion Beam Sources Used in Sputtering,
Journal of Vacuum Science and Technology, Vol. 15, pp 272-276,
March/April 1978 by H. R. Kaufman et al was developed. The Kaufman
ion source is capable of producing beam currents of a large
fraction of an ampere, at energies within the 300-1500 eV range.
The beam is sufficiently controllable, stable and repeatable, to be
satisfactory for use in surface modification devices. A Kaufman ion
source having a grid structure in accordance with the invention can
produce minimum current levels of at least about 200 times and
optimally about 800 to 1500 times the current level of the
Cockraft-Walton and other devices used previously in ion etching.
Such Kaufman ion source beam current is on the order of 30 to 400
mA versus a Cockraft-Walton device beam current of less than 0.3
mA.
The ions used in the U.S. Pat. No. 3,548,189 device are of
substantially the same energy and a uniform current density is
necessary. Only narrow ion beam sources are used and selective
deposition in combination with selective removal is not possible.
Such devices are limited to the figuring of small diameter elements
because beam deflection is used as the steering mechanism, the ion
source not being translatable, i.e., movable. For large diameter
optics, such as those having diameters on the order of one-half
meter or more, the distance from the deflection plates to the
surface would have to be near the diameter of the surface. Beam
current loses due to residual gas in the chamber would be great and
make the process very inefficient. Too, beam dwell pattern
computation is not considered in such prior art devices and methods
using image processing and systems theory for optimized material
removal are not applied.
In devices such as that shown in U.S. Pat. No. 3,699,334, ion beam
impingement control is limited to electrostatic and magnetic
deflection of the beam and to rotation of the object to be etched.
In practicing the invention the ion source is itself moved. The ion
sources used in the prior art are either constructed as an integral
part of the vacuum system containing the object to be etched or
they are external to the vacuum system and connected thereto by a
tube which is evacuated with the vacuum system. No such prior art
systems utilize translatable ion sources. Too, the ion beam is
necessarily maintained continuously in such prior art devices in
part because of the high voltages involved in extracting 20 kV to
100 kV ions. Dwell computations are based on a two step method in
which the symmetrical errors need first be reduced to zero. Then
isolated symmetrical errors are removed. In practicing the
invention all errors, symmetrical and nonsymmetrical, are removed
in one step. Nonsymmetrical and arbitrarily shaped beam objects can
not be figured with such prior art devices. In addition the beam
energies of the prior art devices, 20 to 100 kV, are known to
damage many materials. The apparatus of the invention operates at a
maximum energy of about two kV. The prior art beam taught by the
U.S. Pat. No. 3,699,334 only focuses the ion beam to a diameter
between one and five millimeters whereas that of the invention
focuses the ion beam within a two to five centimeter and larger
range to enable the correction of a wide range of sizes of surface
aberrations for more efficiently than with prior art devices. The
ion source used in accordance with the invention provides electrons
to avoid the electric charge effects requiring a separate source of
electrons in prior art devices.
Thus, it can be seen that the prior art devices and methods can not
figure large surfaces and can not use both removal and deposition
to figure a surface. Such devices are limited to low current, high
energy, narrow beam ion sources and there is no control of beam
current spatial distribution. Large and non-symmetric surfaces can
not be etched by such devices and methods.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided a method and
apparatus for etching a selected surface of an object from its
existing topography to a predetermined topography by comparing the
existing topography to the predetermined topography and using an
algorithm comprising image restoration and an ion beam to
selectively remove material from the surface. The algorithm may
also control deposition to deposit material on the surface to reach
the predetermined topography. The algorithm comprises the use of
nonlinear windowing techniques followed by an image restoration
step to generate a beam dwell array which will yield the desired or
predetermined topography. The method can be used to provide
nonsymmetric as well as symmetric surface topography and to
determine whether an existing surface topography is suitable for
etching or deposition in accordance with preselected citeria. The
ion beam used is preferably high current, low energy and broad.
Surface extrapolations may be used to avoid edge effects. High
curvature and complex surfaces may be formed. An ion source grid
apparatus for extracting ions from a plasma source in accordance
with the invention comprises a sheet of nonconducting vacuum
compatible material, such as a ceramic, having conductive coatings
on each surface. A plurality of apertures penetrate the coatings
and the sheet of material in a predetermined pattern to provide an
ion beam having a preselected spatial distribution when a voltage
is applied across the coatings. The coatings may be disposed in a
plurality of corresponding discrete areas on each side of the sheet
of material, corresponding areas being provided with selected
voltages thereacross to provide an ion beam having a desired
spatial distribution. The voltages may be controllable to change to
spatial distribution. The area and aperture patterns may comprise
concentric circles about a central circular area.
One object of the present invention is to selectively etch the
surfaces of large diameter objects.
Another object of the invention is to control ion beam current
spatial distribution.
Yet another object of the present invention is to use selective
material removal and deposition to figure the surface of an
object.
Still another object of the invention is to provide predetermined
symmetric and nonsymmetric surfaces.
Another object of the invention is to determine which objects are
suitable for etching or deposition in accordance with preselected
criteria.
One advantage of the present invention is that in accordance
therewith, large diameter optics and other elements can be
economically produced.
Another advantage of the present invention is that in accordance
therewith a high current, low energy, broad ion beam can be used to
etch a desired surface configuration on an object.
Yet another advantage of the invention is that surface
extrapolations can be used to avoid edge effects.
Still another advantage is that high surface curvature and complex
surfaces can be figured.
Yet another advantage of the invention is that an object may have
its surface figured, then evaluated for acceptability, and have
subsequent operations such as thin film coating performed thereon
without the object being removed from a vacuum system.
Still another advantage is that delicate and lightweight objects
can be figured because there is no weight loading on the object in
practicing the invention as in conventional grinding or milling
methods.
Additional objects, advantages and novel features of the invention
will be set forth in part in the description which follows, and in
part will become apparent to those skilled in the art upon
examination of the following or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and form a
part of the specification, illustrate the invention, and together
with the description, serve to explain the principles of the
invention. In the drawings:
FIG. 1 illustrates a system for performing the method of the
invention;
FIG. 2 shows a cross section of an ion beam grid structure
representative of those of the prior art;
FIGS. 3A and 3B are cross sectional views of an ion beam grid
apparatus in accordance with the invention;
FIG. 4 illustrates an ion beam grid apparatus in accordance with
the invention having discrete coating areas connectable to separate
voltages;
FIG. 5 is a cross sectional view of the embodiment of FIG. 4
schematically showing the discrete coating areas connected to
different voltage sources;
FIGS. 6A and 6B graphically depict ion beam spatial distribution as
it may be controlled by the embodiment of FIGS. 4 and 5; and
FIGS. 7A and 7B show how ion beam spatial distribution may be
predetermined by aperture placement in the embodiment of FIGS. 3A
and 3B.
DETAILED DESCRIPTION OF THE INVENTION
Reference is now made to FIG. 1 which illustrates a system suitable
for carrying out the method of the invention. As seen therein an
object 10 having a surface 12, beam source 14, an ion source grid
16 and an interferometer or other surface determining or monitoring
instrument 18 such as a phase measuring interferometer or
heterodyne interferometer are positioned within a vacuum chamber
(not shown). A computer 20 is operatively connected to an apparatus
22 for controlling the position of object 10. Beam source 14 and
ion source grid 16 are also under the control of computer 20 as is
monitoring instrument 18. Ion source 14 is preferably a Kaufman ion
source such as disclosed in a publication, Fundamentals of
Ion-Source Operation by Harold R. Kaufman, Library of Congress
Catalog Card Number 84-71750. Kaufman sources are well known and
produce high current, low energy broad ion beams containing nearly
monoenergetic ions so that beam sputtering therewith is essentially
a linear process. The removal profile of the beam from source 14 is
essentially the same regardless of where on the surface 12 of
object 10 beam 24 is directed. Thus, sputtering yield remains
constant. Beam source 14 and/or object 10 is translatable so that
in operation the beam 24 remains normal, or at another selected
fixed angle, to surface 12 or to a reference plane or surface. The
beam 24 does not diverge appreciably and its current energy
distribution remains substantially constant during operation. Beam
source 14 may additionally comprise a sputter magnetron or other
device for ion assisted or other deposition.
In accordance with the invention an algorithm compares a desired
predetermined surface topography with the existing surface
topography of surface 12 on object 10 and utilizing image
restoration, controls beam 24 to ion etch surface 12 to produce the
predetermined surface topography. The predetermined topography may
be symmetric or nonsymmetric and ion assisted deposition may be
also used to figure surface.
In practicing the invention, ion etching and ion depositing beam
figuring is controlled using deconvolution for nearly flat optics,
and matrix computations for optical and other elements having large
surface slopes and high curvatures. The model for figuring
computation begins with the superposition integral, ##EQU1## where
h(x,a,y,B) is the impulse response or point-spread function for the
system model. In this case the impulse is located at point (a,B).
The function f(x,y) is the original surface profile or existing
topography of the optic or other element, and the function d(x,y)
is the desired surface profile or predetermined surface topography.
The function t(a,B) is the time that the ion beam dwells on the
element at point (a,B). The function h(x,a,y,B) is the material
removal profile for the ion beam.
The material removal profile described as h(x,a,y,B) is spatially
variant. The shape of the removal profile can change depending on
where the ion beam is pointed. If the removal profile is found not
to change with position, then the removal profile is said to be
position invariant or spatially invariant. The removal function may
also have additional parameters related to dynamically changing the
mechanical and electromagnetic configuration of the ion source.
Figuring can also be performed by depositing material using a
deposition source. In this case the removal function is replaced
with an addition function which satisfies the same constraints as
the removal function. Those skilled in the art will understand the
calculations of the control parameters using additive techniques,
e.g., ion deposition, are the same as the calculations of the
control parameters for material removal, e.g., ion etching.
For surfaces containing no large slope changes the removal or
deposition profile is spatially invariant for the ion sources used
in practicing the invention. Surfaces having significant curvature
will have spatially variant removal or deposition profiles.
All of the functions but t(a,B) are known. To perform surface
figuring, the function t(a,B) must be calculated. Because surface
figuring in accordance with the preferred embodiment of the
invention is under digital control, metrology and instrumentation,
the integrals of equation 1 are replaced with summations and the
domain is discrete. For the spatially invariant case, the
superposition summation reduces to the definition of the discrete
convolution. ##EQU2##
Equation 2 can be solved using matrix techniques. However, using
orthogonal transforms is easier and provides insight into the
success or failure of the figuring operation.
An orthogonal transform such as the Fourier, Hadamard, Hartley,
Cosine, and the like, has the property of diagonalizing a circulant
(one dimensional) or block-circulant (two dimensional) matrix. This
means that the solution to equation 2 in terms of t(a,B) can be
obtained using algebraic techniques.
Let F be an orthogonal transform. Taking the transform of both
sides of equation 2 gives
where F is the transform of f(x,y), D is the transform of D(x,y),
etc. Equation 3 can be rearranged to form ##EQU3## where g is a
multi-variate function used to control the division when H(u,v)
approaches zero or when F-D becomes noisy. When g=1, equation 4 is
called an inverse filter. When g is a function of the
signal-to-noise ratio at the transform coordinate (u,v), equation 4
is a Least Squares or Wiener filter. The function g can be
optimized to produce a time dwell array t(x,y)=F.sup.-1 F(u,v)
which has optimal or special properties when applied as the time
dwell array for an ion beam figuring operation.
Filters constructed using equation 4 are called restoration,
deblurring or deconvolution filters and are used extensively in
image processing and system controls.
Once all of the functions in equation 2 are known the residuals
arising from the ion figuring process can be calculated by
forming
for different conditions applied with g. E(u,v) is the error
between the desired surface and what can actually be achieved with
a well characterized ion figuring process. In accordance with the
invention, by looking at e(x,y)=F.sup.-1 E(u,v), one can evaluate
the prospects for a successful figuring operation before any work
is actually done. This step provides for the rejection of those
optical or other elements which have surfaces that are economically
or otherwise unsuitable for ion beam figuring by removal or
deposition.
For the case where the ion beam removal or deposition function is
spatially variant, the time dwell array, calculated using matrix
methods, is represented by
where r is a vector formed by stacking the rows of f(x,y)-d(x,y), t
is formed by stacking the rows of t(x,y), and H is formed by
stacking partitions formed by stacking the rows of the point-spread
function h(x,a,y,B) for each (a,B). The matrix H is the spatially
variated point-spread-function (PSF) matrix. The time array is
recovered by forming
where H.sup.-1 is the inverse of the matrix H. When the
point-spread function is spatially invariant, H can be diagonalized
by an orthogonal transform as described previously.
The matrix H is somewhat ill-conditioned, meaning that small
amounts of noise or error present in the matrix coefficients will
have a large effect on the coefficients in the inverse matrix. To
help alleviate this problem, the inverse matrix can be calculated
using Singular Value Decomposition (SVD) or Q-R or other
decompositions where unstable vectors are removed from the inverse
calculation. This produces an approximation to the solution, but
one that has higher tolerance to noise. Iterative constrained
conjugate gradient optimization can also be used to perform the
calculation for the inverse PSF matrix.
The use of the constraints or vector removal corresponds to the use
of g.noteq.1 in equation 4. An estimate of the residuals after
figuring with a well characterized ion beam figuring process can be
found by forming
where H.sup.-1 is the calculated inverse point-spread matrix. The
error vector can then be unstacked to form an error image which can
be inspected for figurability just as in the spatially invariant
case.
Edge effects are produced with conventional figuring techniques due
to the inherent properties of polishing tools. For efficient
material removal a tool should be fairly stiff. As the tool moves
so that part of it extends beyond the edge of the element being
figured, pressure increases on the part of the surface in contact
with the tool and the removal profile distorts. The edges roll over
about the radius of the tool to cause an edge effect. Although many
attempts have been made to solve this problem in conventional
grinding or milling, the effect remains. Similar problems exist in
all types of surface contact tools and devices for material
removal.
In ion beam figuring, removal and deposition profiles do not depend
on mechanical supports and the ion beam profile remains the same
whether or not an object to be figured is in place. Thus, optics
and other elements having essentially no edge effects can be
produced. Because the beam dwell array value at a given point
depends on the condition of the surface in a region around that
point about the width of the removal or deposition function, the
dwell array value depends in part on a condition which does not
exist, since it is off the edge of the element. In practicing the
invention the image of the element provided by the metrology is
treated as a small piece on an infinite surface. Using this model,
the surface of the element is imagined to be a snapshot through an
aperture of the surface map of a much larger element extending far
beyond the field of view of the metrological instrumentation. Data
is constructed to fill in those parts of the surface map which
would correspond to those parts of the larger element obscured by
the aperture. Hence the image restoration or matrix solutions see
an element with no edges and compute correct dwell array for such
an element. The constructed data must have the same properties in
terms of surface structure as the original element because there
should be a match of the real data with the nonphysical data at the
edge of the element.
Construction of nonphysical data is achieved with Band Limited
Surface Extrapolation (BLSE) using orthogonal transforms. Original
data is filtered to provide a smoothed result with some data
outside the original data. The original data is then reinserted
into the resultant image. These steps are repeated a number of
iterations to build up data outside the original data, limited in
frequency content by the filter which provides the band limits.
Convergence can be slow, even converging an over infinite
iterations. Since ideal filters introduce "ringing" artifacts into
the image, variable order filters such as Butterworth, Chebyshev,
or other more advanced filters can be used. In practicing the
invention, the cutoff frequency of the filter is varied during the
progressive iterations, typically proceeding from higher bandwidths
to lower bandwidths, with the final iterations being performed
using the transform of the ion beam removal or deposition function
as the filter. The ion beam removal or deposition function is the
ultimate filter because it eliminates any frequencies not present
in the ion beam itself, alleviating restoration difficulties in
equation 4.
To further speed the convergence, the filters are set during early
iterations to amplify, in some cases nonlinearly, some of the
frequencies in the pass band. This builds up the nonphysical data
area more quickly than when conventional normalized filters are
used.
Additional gains in edge smoothness are obtained in some cases by
offsetting the optical surface with respect to its reference plane.
This costs additional figuring time during which the centroid of
the ion beam is mostly off the surface of the element being
figured. However, this produces higher quality edge figuring.
The invention is applicable to the production of large optical or
other surfaces due to its inherent scalability. As the size of a
work piece is increased, ion beam current can be increased by using
larger ion sources or by using a plurality of small ion sources
which can be run simultaneously. The use of a plurality of sources
reduces the time needed to figure a particular surface and
distributes the thermal load across the surface of the element
during figuring to thereby reduce thermally induced distortion. The
plurality of sources may all be of the same size or more likely, of
different selected sizes to minimize element figuring time. The use
of several size ion sources also provides figuring over large
spatial frequency bands which results in a better final surface
figure. Spatial ion beam current density can be dynamically tuned
using single or plural sources in practicing the invention to
provide an optimal final surface figure.
Because weight loading due to gravity and forces applied in
conventional figuring techniques and mechanical distortion caused
by polishing tool weight are eliminated, very light weight and
flexible optical and other elements can be figured using the
invention.
Referring now to FIG. 2 which shows a typical prior art Kaufman ion
source grid structure for ion beam generation, as seen therein,
walls 30 and 32 contain a plasma 34 as a source of ions. A pair of
grids 36 and 38 of metal or other suitable conductive material
comprising perforated sheet stock are spaced apart with insulators
40 and disposed on extensions 42 and 44 of walls 30 and 32,
respectively. Ions as indicated by the arrows are accelerated from
the plasma through the performations in the grid by appropriate
voltage applied across the grids 36 and 38. As will be appreciated
by those skilled in the art, the grid is rather delicate and
readily subject to misalignment, damage and disablement because it
is dependent on the strength of the metal or other conductive
material for structural integrity. Too, this prior art grid
apparatus is limited to a single voltage differential in that its
conductive material can not be separated into discrete, spaced,
corresponding areas to which different voltages can be applied, as
in practicing the instant invention.
FIGS. 3A and 3B are cross sectional views of an ion grid apparatus
in accordance with the invention showing a nonconductive sheet 50
which may, for example, comprise a ceramic or other vacuum
compatible nonconductive material and which is preferably about
0.025 inch to about 0.020 inch thick. The surfaces of the sheet 50
are covered with conductive layers 52 and 54 which may comprise
electroplated coatings, metal foil, vacuum deposited coatings, and
the like. In practicing the invention, those skilled in the art
will recognize suitable metals or other conductive materials for
constructing the layers and suitable techniques for their
fabrication. FIG. 3A shows a layered sheet before apertures are
drilled or otherwise provided, whereas FIG. 3B shows the structure
of FIG. 3A with apertures placed therein. The invention as shown in
FIG. 3B functions with a plasma source and the sheet 50 may be
simply fastened to the ends of the walls of the source.
The FIG. 3B embodiment may comprise perforations in any selected or
desired pattern in order to provide an ion beam of a predetermined
spatial distribution. For example, FIG. 7A shows an essentially
Gaussian distribution of beam current density achieved with uniform
aperture spacing on the grid structure. FIG. 7B shows how spatial
distribution can be varied by varying aperture spacing. The graph
depicts beam current density for the grid aperture spacing
schematically illustrated below its X axis.
Spatial distribution of beam current density can be selectively
varied in accordance with the invention using the grid apparatus
shown in FIG. 4 which shows a conductive layer disposed on one side
of a sheet 56 in separate and discrete areas. The other side of
sheet 56 contains a similar corresponding pattern of areas of
conductive layer. Thus, the conductive layer on each side of the
sheet comprises separate and discrete areas corresponding to, and
essentially mirror images of, the separate and discrete areas on
the opposite side of the sheet. As seen therein, a sheet 56
contains separate areas 57, 58, 59, 60, 61 and 62 of conductive
layer. Conductive leads 63, 64, 65, 66, 67, and 68 connect areas
57-62 to different sources of voltages as seen in FIG. 5 which
schematically shows voltage sources 70, 72 and 74 connecting
through appropriate leads to provide separate, preferably
individually controllable, voltage potentials across corresponding
areas on each side of the sheet. Thus, the various conductive areas
and their opposing mirror image areas are operable at corresponding
voltages. The various voltages may be the same or different,
depending on a particular operation. The voltages may be
preselected and fixed or they may be variable either manually by an
operator or under automatic control. FIGS. 6A and 6B show beam
current density as a function of voltage for the FIG. 5 embodiment.
In FIG. 6A, voltages V1, V2 and V3 are all 500 volts. FIG. 6B shows
beam current density for V1 and V2 at 500 volts and V3 at zero
volts. The circular central area 62 surrounded by concentric
circles 61-57 is for purpose of illustration of a preferred
embodiment and area configuration is not limited thereto.
Similarly, the number of sets of corresponding areas shown are for
illustration only, and any number of one or more sets of
corresponding areas may be used in practicing the invention. The
invention is additionally not limited to the aperture distribution
shown for purposes of illustration only, and those skilled in the
art will recognize other aperture patterns can be used to practice
the invention. Those skilled in the art will also recognize
voltages, grid apparatus size and aperture size suitable for use in
particular applications in order to practice the invention.
The various grid structures described can be used to practice the
method of the invention, which is particularly suitable for
figuring the surfaces of large optics and other elements.
In practicing the invention, after a surface is configured, it may
be coated with an additional material. Thus, the invention can be
used to manufacture a mirror by etching or depositing material to
figure a surface and then coating the surface with a reflective
coating. Similarly, a nonreflective or other coatings may be added
to an element figured in accordance with the invention. The method
of the invention is particularly useful to coat elements which are
damaged when heated, since many conventional coating techniques
require the substrate be heated to from 150.degree. to 300.degree.
C. Ion assisted deposition of coatings may be carried out using a
magnetron or other such device. Too, ion etching can be applied to
a coated element to alter coating thickness to provide a desired
coating thickness profile.
The invention has been described in detail with particular
reference to preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the scope of the invention.
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